Many hydrochlorocarbons and hydrochlorofluorocarbons (HCFC's) can be converted to hydrofluorocarbons (HFC's) or other HCFC's by direct hydrofluorination, liberating the chlorine from the molecule in the form of HCl. These reactions can use any one of a number of different reactor designs, and can take place in a gas or liquid phase. Gas phase reactions usually have higher selectivity and are well suited to continuous flow systems. The advantages of continuous flow reactors over batch systems from a reactor productivity perspective are well known. In the reactor systems involving HFC's, pressurized reactors have the advantage of allowing the distillation train for product and HCl recovery to be run under pressure, whereby the need for refrigeration is reduced.
One such HFC is 1,1,1-trifluoroethane (R143a), which can be synthesized by hydrofluorination of 1-chloro-1,1,-difluoroethane (R142b). This reaction can take place either in the gas phase or liquid phase. It is well suited to a continuous flow reactor system. In a gas phase reactor, a catalyst is typically required. The reaction is considerably exothermic, so that if the reaction is run under isothermal or near isothermal conditions, substantial cooling is required to take away the heat of reaction.
Another hydrofluorination of interest is that of 1,1,1,2-tetrafluorochloroethane (R124) or 2,2-dichloro-1,1,1-trifluoroethane (R123) to pentafluoroethane (R125). In U.S. Pat. No. 5,334,787 there is described a gas phase reaction for such a system which is near isothermal.
In the design of continuous flow reactor systems, the advantages of plug flow over backmixed or stirred systems is well known. The plug flow reactor is kinetically more efficient and therefore permits higher productivity per unit volume. Furthermore, within the subset of plug flow reactors, the adiabatic reactor has obvious economic advantages over a cooled reactor. Since there is no heat removal in an adiabatic reactor, it can utilize a simple design of a pipe having virtually any diameter. By contrast, a cooled reactor is usually designed as a "shell and tube" reactor having cooling medium on the outside of small diameter tubes, which contain the process fluids and the catalyst. In this type of reactor configuration, changing catalyst can be time consuming and costly. Furthermore, slight differences in the way the catalyst is packed from tube to tube can cause varying pressure drops which result in maldistribution of the feeds among the tubes. For an adiabatic reaction system to work, the catalyst must be selective over a broad range of temperatures. For R143a, this appears to be especially true since literature sources indicate the heat of reaction to be as high as 19 kcal/mol.:
______________________________________ .DELTA.H.sub.f Compound kcal/mol Reference ______________________________________ HF -64.8 1 R142b -116.3 2 R143a -178.2 1 HCl -22.1 1 ______________________________________
Reference 1 is: Reid, R. C., Prausnitz, J. M., and Sherwood, The Properties of Gases and Liquids, Third Ed. McGraw-Hill, 1977; Reference 2 is: Benson, S. W. et al., "Additivity Rules for the Estimation of Thermochemical Properties," Chem. Review, 69:279 (1969).
Another important consideration for any gas phase hydrofluorination is the effect of hydrogen fluoride (HF) association. Hydrogen fluoride in both the gas and liquid phase is thought to associate with itself to form an oligomer: (HF).sub.n, n&gt;1. The equilibrium between monomer and oligomer is a function of temperature and pressure. For a given pressure, disassociation (sometimes called depolymerization) occurs with increasing temperature. Since this disassociation is considerably endothermic, HF has a very high apparent heat capacity. An illustration of the effects of depolymerization is seen in FIG. 1, which is an enthalpy chart for HF. This chart is taken from Yarboff, R. M., Smith, J. C., and Lightcap, E. H., "Thermodynamic Properties of HF", Journal of Chemical Engineering, Vol. 9, No. 2, 179, April, 1964; Enthalpy is a measure of the heat content of a given system. The straight line 101 at the top of the chart represents the ideal gas region. At the bottom of the chart is the saturation curve 102. The vertical distance between the saturation curve and the ideal gas line represents the heat of depolymerization. The tie lines (e.g., 103) connecting the ideal gas line and the saturation curve provide an estimate of HF enthalpy in the region where HF is partially associated. The relative amount of association can be approximated by the location on the tie line connecting the saturation curve to the ideal gas line. The enthalpy changes occurring in hydrogen fluoride are critical to the design of an adiabatic hydrofluorination reactor.
Other hydrofluorination reactions are thought to behave similarly to the R142b hydrofluorination, for example, the reaction of R124 (1,1,1,2-tetrafluoro-2-chloroethane) to R125 (pentafluoroethane). It is an exothermic reaction (estimated to be about -10 kcal/mol). R124 has a similar boiling point to R142b and has a low boiling azeotrope with HF as does R142b. The dew points of HF mixtures would be expected to be very similar for R124 and R142b.